1. Field of the Invention
This invention relates to compositions and processes for depositing a manganese containing phosphate conversion coating on metal surfaces, particularly the surfaces of ferrous metals, galvanized steel, and other predominantly zinciferous surfaces. The invention particularly relates to such compositions and processes that produce, at a temperature not more than 80.degree. C., a conversion coating suitable as a high quality undercoat for paint and like organic coatings.
2. Statement of Related Art
The general process of phosphate conversion coating is well known in the art. See, e.g., M. Hamacher, "Ecologically Safe Pretreatments of Metal Surfaces", Henkel-Referate 30 (1994), pp. 138-143, which, except to the extent that it may be contrary to any explicit statement herein, is hereby incorporated herein by reference. In brief, contact of active metals such as iron and zinc with aqueous acidic compositions containing a sufficient concentration of phosphate ions results in the deposition on the active metal surfaces of a conversion coating containing phosphate ions and some metallic cations, which are derived from dissolution of the active metal being phosphate conversion coated, from metallic cations with a valence of at least two that are present in the aqueous acidic compositions, or both. In many instances, particularly when the phosphating compositions contain zinc, nickel, or manganese, in order to speed the process and improve the uniformity of the coating, it is customary to include in the coating composition a component called an "accelerator" that does not usually become incorporated into the coating formed. Typical widely used accelerators include nitrite and chlorate ions, water soluble nitroaromatic organic compounds such as p-nitrobenzene sulfonic acid, and hydroxylamine, the latter almost always in the form of salts or complexes and different from most other accelerators because, in the concentrations in which it is normally used, it is not a strong enough oxidizing agent to oxide Fe(II) ions to Fe(III) ions, one of the functions of most other accelerators.
Prior art phosphating compositions that include manganese as substantially the only metal cations with a valence of two or more in the compositions have been known and used. However, such compositions have been previously used in practice only at relatively high temperatures, almost always above 80.degree. C. and more often above 88.degree. C. Such compositions have been notoriously prone to sludging, a phenomenon that occurs with almost all phosphate conversion coating compositions but is quantitatively aggravated when the compositions contain manganese as the predominant cations with a valence of two or more.
Furthermore, such prior art manganese based conversion coatings have been normally used only to deposit heavy and usually amorphous conversion coatings that function primarily as lubricant carriers during cold working of the metal objects underlying the conversion coatings formed. Although this is one important practical application of phosphating, providing an undercoat for paints is still more important and in the past has not been advantageously accomplished by phosphating compositions in which divalent manganese ions were substantially the only metal cations present with a valence of two or more: The thick phosphate conversion coatings readily achieved with manganese phosphating compositions are too thick and/or brittle to provide good adhesion to subsequently applied paint and like materials, presumably because thick manganese phosphate coatings are readily cracked by even fairly small mechanical shocks. On the other hand, controlling manganese phosphating compositions to produce the thinner, usually microcrystalline types of phosphate conversion coatings that do provide good adhesion to subsequently applied paint and are readily produced by phosphating compositions that contain zinc, nickel, cobalt, and/or iron in a total amount of at least 0.5 grams per liter (hereinafter usually abbreviated as "g/L") has proved practically difficult if not impossible with manganese phosphating compositions.
Also, prior art manganese phosphating compositions are not known to have produced satisfactory quality conversion coatings when contacted with the surfaces to be coated by spraying only, and have generally been used only when the surfaces to be coated were immersed in the compositions.